We should have completed the Ronaldo deal earlier, once United noticed him we had no chance.
Suarez obviously is the biggest miss ever, and I agree with Xabi, but 2 other names I will throw out here:
Di Maria twice, once when he was in Argentina and then when Madrid wanted to sell him. He failed at United because LVG didn't know how to use him but would have absolutely thrived under Wenger. Add to the fact if we got him, we wouldn't have gotten Ozil so double win for us.
Mbappe. Wenger should have pushed harder and gotten him at all cost.